1. Technical Field
The present disclosure relates to imaging devices. More particularly, it relates to a circuit and method for measuring toner or ink levels in the imaging unit of an imaging device.
2. Description of the Related Art
Image forming devices such as copiers, laser printers, facsimile machines and the like typically use one or more toner containers to hold toner supply used for image forming processes. In some image forming devices, a large toner supply is provided in a reservoir in a toner cartridge that mates with a separate imaging unit. The imaging unit may include a sump that holds a smaller amount of toner, enough to ensure toner is adequately supplied by a toner adder roll and a developer roll to a photoconductive drum. As toner within the imaging unit sump is depleted due to printing operations, additional toner is transferred from the toner cartridge to the imaging unit sump.
To ensure satisfactory operation of the imaging unit to transfer toner, the toner level within the imaging unit sump is maintained at a proper level. For example, if the imaging unit sump holds too much toner, toner may pack in the imaging unit sump, leak out of the ports and eventually break other components located inside and outside the imaging unit. If the toner level in the imaging unit sump gets too low, the toner adder roll may starve, causing a doctor blade of the imaging unit to film and damage the developer roll which may eventually impair the future performance of the imaging unit. As such, it is desirable to know the toner level in the imaging unit sump so as to effectively determine when to move toner from toner cartridge to the imaging unit sump.
Some methods for determining toner level in a container use estimates of toner use and accumulation based on print or time counts. However, these methods may not be accurate due to variability in factors such as the environment, developer roll age, toner patch sensing cycles, and toner transfer parameters.
Other known techniques for sensing or determining toner level include the use of electrical sensors that measure the motive force required to drive an agitator within a toner container, optical devices including mirrors and toner dust wipers in a container, and other opto-electromechanical devices such as a flag that moves with the toner level to actuate a sensor that triggers only when the volume reaches a predetermined level. Unfortunately, the addition of moving hardware increases component complexity and opportunities for errors. For instance, toner agitation may create unwanted toner dust in addition to the added complication of moving hardware.
Other techniques for sensing or determining toner level include use of a capacitive sensor disposed within a toner container, such as a waste toner container, and circuitry for sensing the capacitance of the capacitive sensor as toner levels in the container change. In one existing implementation, illustrated in FIG. 4, an AC signal generator 101 is connected to the capacitor Cx to be measured and applies a generally square wave signal thereto. Capacitor Cx couples the AC signal generator 101 to a high-pass amplifier 102 which buffers and amplifies the AC square wave signal. A synchronous rectifier 103, which is coupled to the output of the high pass amplifier 102, operates at the same frequency as AC signal generator 101 and is synchronized thereto. Synchronous rectifier 103 converts the square-wave (bipolar) signal into a unipolar signal. A low-pass filter 104 receives the unipolar signal from synchronous rectifier 103 and amplifies and smoothes the unipolar signal. Low pass filter 104 receives the filtered, unipolar signal and drives a DC output voltage Vout.
Output voltage Vout is modulated by modulator circuit 105 to a square wave at the frequency of AC signal generator 101 and synchronized thereto. The output of modulator circuit 105 is fed through a reference capacitor Cref back to the input of high pass amplifier 102. Modulator circuit 105 inverts the phase of the signal from AC signal generator 101 so that modulator circuit 105 and the AC signal generator 101 are 180 degrees out of phase with each other. The feedback loop controls output voltage Vout such that the input to high pass amplifier 102 is effectively a DC signal. In other words, the AC current through capacitor Cx is substantially balanced by the current through reference capacitor Cref. The transfer function for this circuit is:Vout=VAC*Cx/Cref,where VAC is the voltage output of AC signal generator 101. With Vout, VAC and Cref being known values, the capacitance of capacitor Cx can be determined which is indicative of the amount of toner existing in the toner container in which capacitor Cx is disposed.
The circuit of FIG. 1 requires both terminals of capacitor Cx to float relative to ground. One terminal is referred to as the driven terminal and is connected to AC signal generator 101. The other terminal is the sense terminal and is connected to reference capacitor Cref and high-pass amplifier 102. Though the circuit of FIG. 1 has been used to effectively sense toner levels in a waste toner container by solving for capacitor Cx in the above equation, the circuit is not as robust or cost effective when used in some capacitive toner level sensing applications by, for example, requiring use of flexible circuits or the like to form the plates of capacitor Cx. Accordingly, there is a need for a more robust capacitance based toner level sensing circuit and method.